The present invention relates to a method for cleaning unbrazable metal parts and, more particularly, to a process for cleaning metal parts with an atmosphere containing the elements C-O-H-F so that the parts can thereafter be brazed or otherwise bonded.
Late model gas turbine engines for example, those on the Boeing 747, the DC-10, and the Lockheed L-1011, employ in their turbine sections nickel based alloys that are gamma prime hardened. Many other engines contain such materials, and the use of gamma prime hardened alloys will no doubt increase in the future due to the desirable properties of these superalloys. Engine parts made of such alloys are very expensive and, at present, are not repairable when crack damaged due to metal fatigue. Attempts to weld repair such components results in post-weld cracking. Likewise, conventional brazing cannot be effected because nickel-base braze alloys will not run on the gamma prime hardened surfaces. Thus, although a molten brazing alloy under high vacuum might stick new gamma prime hardened parts together, it has not previously been possible to place brazing alloy inside cracks in damaged gamma prime hardened alloy parts. The control of the parameters of that process to enable low temperature cleaning of these parts is not however, disclosed in U.S. Pat. No. 4,188,237.
A successful braze is manifest when braze material is placed at the source of a crack (say 0.001 inch wide and one-half inch long) and, at brazing temperature, not only melts and sticks to the parent material, but also runs into and fills the length of the crack. Apparently, in use a gamma prime hardened alloy becomes oxidized and/or sulfidized to the extent that the aluminum, titanium and chromium oxides or sulfides, which coat the surface of the part, including the surfaces of the crack, prevent successful repair by brazing.
Accordingly, it has been recognized that such parts must be cleaned if they are to be brazed. Keller et al, in U.S. Pat. No. 4,098,450 suggests the use of chromium fluoride (CrF.sub.3) and hydrogen (H.sub.2) to clean damaged parts of gamma prime hardened alloys prior to a braze repair. From that process description, it is speculated that the following reactions comprise the suggested mechanism: EQU 2(CrF.sub.3.3-1/2H.sub.2 O+3H.sub.2 .fwdarw.6HF+2Cr+7H.sub.2 O (1) EQU MO.sub.x +2xHF.fwdarw.MF.sub.2x +xH.sub.2 O (2)
If then, MF.sub.2x is volatile at the reaction temperature, the oxide is effectively reduced and the base metal (M) should be brazable. It is readily shown, however, that reaction (1) is incapable of producing an HF concentration in the gas in excess of that which is in equilibrium with chromium and its fluoride. Such concentrations are incapable of causing reaction (2) to proceed as shown when the metal oxide is more noble than Cr.sub.2 O.sub.3. In particular, if the metal is aluminum, the reaction will not proceed and cracks in gamma prime hardened alloys will not be cleaned. Because of this and inadequate control over the H/O ratio, uniform reproducibility of results are lacking and many parts cleaned by this process are still not brazable.
A much more effective process is that disclosed and claimed in U.S. Pat. No. 4,188,237. However, that patent is for the most part directed to cleaning crack-damaged gamma prime hardened alloys. It has been established that a similar process is also effective in cleaning other metals, especially stainless steels, superalloys, and solid solution superalloys, as well as the gamma prime hardened nickel alloys.
Fabrication of stainless steel composites by nickel brazing has long been of commercial interest. Before such devices can be fabricated by brazing, however, it is necessary to clean the faying surfaces of all metal oxides (or other compounds). The problem arises because the surfaces of such alloys are covered with a passive film which will not be wetted by a brazing alloy. The most stable oxide in such a film in these alloys is that of chromium and any pre-braze cleaning technique necessarily centers on this compound. It is necessary to reduce the chromium oxide (and all other oxides) to its metallic element before brazing can be accomplished.
A technique that is commonly employed to prepare such alloys for brazing is that of exposing them to a dry hydrogen atmosphere at high temperatures (&gt;1000.degree. C.). Hydrogen cleaning is highly functional but has the disadvantage that the cleaning only takes place at temperatures that are near to or higher than the brazing temperature. It is also known that stainless steel can be brazed in a stable reducing atmosphere of fluoride. In a paper presented by the Toulouse, France, Microturbo Company representatives at the American Welding Society (AWS) meeting in Philadelphia in April 1977, later published in the Welding Journal, November, 1977, and entitled "Brazing Stainless Steel in a Stable Reducing Atmosphere of Fluoride," there is described a brazing process carried out in a halogen atmosphere obtained by the decomposition of fluorine salts such as ammonium bifluoride acid and chromium fluoride. The proposed reactions are: EQU NH.sub.4 F HF+Cr.fwdarw.CrF.sub.2 +NH.sub.3 +H.sub.2 ( 1) EQU NH.sub.3 .fwdarw.1/2N.sub.2 +3/2H.sub.2 (on contact with metal) (2) EQU CrF.sub.2 +H.sub.2 .fwdarw.2HF+Cr (3) EQU 6HF+Cr.sub.2 O.sub.3 .fwdarw.2CrF.sub.2 +3H.sub.2 O+F.sub.2 ( 4)
There are two pertinent observations regarding these reactions: (a) the object would appear to be the production of HF gas which, in turn, does the cleaning, and (b) any elemental fluorine that forms is produced downstream of the work piece (see reaction 4). It is noted that in the presented paper there is an indication that "the technique cannot be used on assemblies of materials having a high level of electropositivity, such as titanium and zirconium", and "it is essential to avoid the introduction of carbon into the furnace during brazing".
Similarly, Moore in U.S. Pat. No. 2,585,819 discloses a process of fluxing metal parts with a stable, non-oxidizing atmosphere containing HF gas. The metal parts are ones such as steels which are to be brazed or soldered.
Finally, Low, U.S. Pat. No. 2,851,387 relates to a process for nitriding high chromium stainless steels. In Low's discussion of the prior art he notes that all prior processes of depassifying such steels require immediate nitriding or the internal effects of the depassifying were lost. A specific purpose of his invention is a combined reactivating and nitriding operation which avoids any problems of interruption in the sequence of steps. The combined operations are provided by a mixture of decomposed fluorocarbon resin gases and ammonia gas. The result is a nitrided product; a cleaned brazable product is not produced.